1. Field of the Invention
This invention relates to a dental handpiece and more particularly to a dental handpiece in which an increase in the cutting torque of the handpiece is made possible by applying a simple modification to turbine blades. The range of application of this invention includes a type wherein the turbine rotor is journaled by air bearings in a thrust gap and a radial gap and a type wherein the turbine rotor is mechanically journaled by ball bearings.
2. Description of the Prior Art
When a load contrary to the direction of rotation of a tool shaft of a handpiece is applied to the tool shaft (typically, when tooth cutting is carried out), a handpiece of the type wherein the rotor is rotated at high speed is caused to stop rotation of the tool shaft at a relatively low load to thereby make cutting impossible. This fact will now be described by way of two examples below.
(1) When an inverse load is progressively applied to an air-driven turbine operated air bearing journaled type handpiece which is being rotated at a speed of 500,000 rpm, the handpiece is reduced in the number of rotations in proportion as the load is increased as shown in curve a in FIG. 6; and when it is subjected to a load of 100 g, the handpiece stops rotation and it is impossible to cut.
(2) On the other hand, although not shown, an air-driven mechanical (ball bearing) journal mechanism directed mainly toward medium-speed cutting (300,000-400,000 rpm), smaller in the number of rotations, stops in the range of 350,000-300,000 rpm when a load of 230-250 g is applied.
In an attempt to solve those drawbacks, a number of proposals have been made. U.S. Pat. No. 3,055,112 discloses a dental handpiece having an air turbine consisting of twin rows of rotor vanes on either side of a centrally disposed annular pressurized chamber. However, it does not provide blades whose width is longer than that of the fluid supplying opening, whereby the torque thus obtained is limited.
U.S. Pat. No. 25,964 (Reissue) discloses a dental handpiece whose blades have saw-like teeth thereon. As a result, some portion of the air supplied through the opening will pass through the space between the teeth causing reduction in originating torque.
U.S. Pat. No. 4,146,964 discloses a dental handpiece including a pair of oppositely facing hollow caps having turbine blades around their peripheries arranged in a staggered relationship. This dental handpiece has apertures in the middle of the blade; however, such blade is not provided with opening cuts and flanges as a single unit. Therefore, the apertures taught by U.S. Pat. No. 4,146,964 does not have much to do with the efficiency of the torque.